OpenGL Shading Methods: Per-Vertex vs Per-Fragment, Polygonal Shading, Flat, Smooth, Gouraud

CS 480/680 Computer Graphics Shading in OpenGL Dr. Frederick C Harris, Jr.

Objectives • Introduce the OpenGL shading methods • per vertex vs per fragment shading • Where to carry out • Discuss polygonal shading • Flat • Smooth • Gouraud

OpenGL shading • Need • Normals • material properties • Lights • State-based shading functions have been deprecated (glNormal, glMaterial, glLight) • Get computer in application or send attributes to shaders

Normalization • Cosine terms in lighting calculations can be computed using dot product • Unit length vectors simplify calculation • Usually we want to set the magnitudes to have unit length but • Length can be affected by transformations • Note that scaling does not preserved length • GLSL has a normalization function

Normal for Triangle p1 n p2 plane n·(p - p0 ) = 0 n = (p2 - p0 ) ×(p1 - p0 ) p p0 normalize n  n/ |n| Note that right-hand rule determines outward face

Specifying a Point Light Source • For each light source, we can set an RGBA for the diffuse, specular, and ambient components, and for the position vec4 diffuse0 =vec4(1.0, 0.0, 0.0, 1.0); vec4 ambient0 = vec4(1.0, 0.0, 0.0, 1.0); vec4 specular0 = vec4(1.0, 0.0, 0.0, 1.0); vec4 light0_pos =vec4(1.0, 2.0, 3,0, 1.0);

Distance and Direction • The source colors are specified in RGBA • The position is given in homogeneous coordinates • If w =1.0, we are specifying a finite location • If w =0.0, we are specifying a parallel source with the given direction vector • The coefficients in distance terms are usually quadratic (1/(a+b*d+c*d*d)) where d is the distance from the point being rendered to the light source

Spotlights • Derive from point source • Direction • Cutoff • AttenuationProportional to cosaf f q -q

Global Ambient Light • Ambient light depends on color of light sources • A red light in a white room will cause a red ambient term that disappears when the light is turned off • A global ambient term that is often helpful for testing

Moving Light Sources • Light sources are geometric objects whose positions or directions are affected by the model-view matrix • Depending on where we place the position (direction) setting function, we can • Move the light source(s) with the object(s) • Fix the object(s) and move the light source(s) • Fix the light source(s) and move the object(s) • Move the light source(s) and object(s) independently

Material Properties • Material properties should match the terms in the light model • Reflectivities • w component gives opacity vec4 ambient = vec4(0.2, 0.2, 0.2, 1.0); vec4 diffuse = vec4(1.0, 0.8, 0.0, 1.0); vec4 specular = vec4(1.0, 1.0, 1.0, 1.0); GLfloat shine = 100.0

Front and Back Faces • Every face has a front and back • For many objects, we never see the back face so we don’t care how or if it’s rendered • If it matters, we can handle in shader back faces not visible back faces visible

Emissive Term • We can simulate a light source in OpenGL by giving a material an emissive component • This component is unaffected by any sources or transformations

Transparency • Material properties are specified as RGBA values • The A value can be used to make the surface translucent • The default is that all surfaces are opaque regardless of A • Later we will enable blending and use this feature

Polygonal Shading • In per vertex shading, shading calculations are done for each vertex • Vertex colors become vertex shades and can be sent to the vertex shader as a vertex attribute • Alternately, we can send the parameters to the vertex shader and have it compute the shade • By default, vertex shades are interpolated across an object if passed to the fragment shader as a varying variable (smooth shading) • We can also use uniform variables to shade with a single shade (flat shading)

Polygon Normals • Triangles have a single normal • Shades at the vertices as computed by the Phong model can be almost same • Identical for a distant viewer (default) or if there is no specular component • Consider model of sphere • Want different normals at each vertex even though this concept is not quite correct mathematically

Smooth Shading • We can set a new normal at each vertex • Easy for sphere model • If centered at origin n = p • Now smooth shading works • Note silhouette edge

Mesh Shading • The previous example is not general because we knew the normal at each vertex analytically • For polygonal models, Gouraud proposed we use the average of the normals around a mesh vertex n = (n1+n2+n3+n4)/ |n1+n2+n3+n4|

Gouraud and Phong Shading • Gouraud Shading • Find average normal at each vertex (vertex normals) • Apply modified Phong model at each vertex • Interpolate vertex shades across each polygon • Phong shading • Find vertex normals • Interpolate vertex normals across edges • Interpolate edge normals across polygon • Apply modified Phong model at each fragment

Comparison • If the polygon mesh approximates surfaces with a high curvatures, Phong shading may look smooth while Gouraud shading may show edges • Phong shading requires much more work than Gouraud shading • Until recently not available in real time systems • Now can be done using fragment shaders • Both need data structures to represent meshes so we can obtain vertex normals

Vertex Lighting Shaders I // vertex shader in vec4 vPosition; in vec3 vNormal; out vec4 color; //vertex shade // light and material properties uniform vec4 AmbientProduct, DiffuseProduct, SpecularProduct; uniform mat4 ModelView; uniform mat4 Projection; uniform vec4 LightPosition; uniform float Shininess;

Vertex Lighting Shaders II void main() { // Transform vertex position into eye coordinates vec3 pos = (ModelView * vPosition).xyz; vec3 L = normalize( LightPosition.xyz - pos ); vec3 E = normalize( -pos ); vec3 H = normalize( L + E ); // Transform vertex normal into eye coordinates vec3 N = normalize( ModelView*vec4(vNormal, 0.0) ).xyz;

Vertex Lighting Shaders III // Compute terms in the illumination equation vec4 ambient = AmbientProduct; float Kd = max( dot(L, N), 0.0 ); vec4 diffuse = Kd*DiffuseProduct; float Ks = pow( max(dot(N, H), 0.0), Shininess ); vec4 specular = Ks * SpecularProduct; if( dot(L, N) < 0.0 ) specular = vec4(0.0, 0.0, 0.0, 1.0); gl_Position = Projection * ModelView * vPosition; color = ambient + diffuse + specular; color.a = 1.0; }

Vertex Lighting Shaders IV // fragment shader in vec4 color; void main() { gl_FragColor = color; }

Fragment Lighting Shaders I // vertex shader in vec4 vPosition; in vec3 vNormal; // output values that will be interpolatated per-fragment out vec3 fN; out vec3 fE; out vec3 fL; uniform mat4 ModelView; uniform vec4 LightPosition; uniform mat4 Projection;

Fragment Lighting Shaders II void main() { fN = vNormal; fE = vPosition.xyz; fL = LightPosition.xyz; if( LightPosition.w != 0.0 ) { fL = LightPosition.xyz - vPosition.xyz; } gl_Position = Projection*ModelView*vPosition; }

Fragment Lighting Shaders III // fragment shader // per-fragment interpolated values from the vertex shader in vec3 fN; in vec3 fL; in vec3 fE; uniform vec4 AmbientProduct, DiffuseProduct, SpecularProduct; uniform mat4 ModelView; uniform vec4 LightPosition; uniform float Shininess;

Fragment Lighting Shaders IV void main() { // Normalize the input lighting vectors vec3 N = normalize(fN); vec3 E = normalize(fE); vec3 L = normalize(fL); vec3 H = normalize( L + E ); vec4 ambient = AmbientProduct;

Fragment Lighting Shaders V float Kd = max(dot(L, N), 0.0); vec4 diffuse = Kd*DiffuseProduct; float Ks = pow(max(dot(N, H), 0.0), Shininess); vec4 specular = Ks*SpecularProduct; // discard the specular highlight if the light's behind the vertex if( dot(L, N) < 0.0 ) specular = vec4(0.0, 0.0, 0.0, 1.0); gl_FragColor = ambient + diffuse + specular; gl_FragColor.a = 1.0; }

OpenGL Shading Methods: Per-Vertex vs Per-Fragment, Polygonal Shading, Flat, Smooth, Gouraud